Declaring Attributes of Functions
=================================
In GNU C, you declare certain things about functions called in your
program which help the compiler optimize function calls and check your
code more carefully.
The keyword `__attribute__' allows you to specify special attributes
when making a declaration. This keyword is followed by an attribute
specification inside double parentheses. Eight attributes, `noreturn',
`const', `format', `section', `constructor', `destructor', `unused' and
`weak' are currently defined for functions. Other attributes, including
`section' are supported for variables declarations (*note Variable
Attributes::.) and for types (see Type Attributes.).
You may also specify attributes with `__' preceding and following
each keyword. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use `__noreturn__' instead of `noreturn'.
`noreturn'
A few standard library functions, such as `abort' and `exit',
cannot return. GNU CC knows this automatically. Some programs
define their own functions that never return. You can declare them
`noreturn' to tell the compiler this fact. For example,
void fatal () __attribute__ ((noreturn));
void
fatal (...)
{
... /* Print error message. */ ...
exit (1);
}
The `noreturn' keyword tells the compiler to assume that `fatal'
cannot return. It can then optimize without regard to what would
happen if `fatal' ever did return. This makes slightly better
code. More importantly, it helps avoid spurious warnings of
uninitialized variables.
Do not assume that registers saved by the calling function are
restored before calling the `noreturn' function.
It does not make sense for a `noreturn' function to have a return
type other than `void'.
The attribute `noreturn' is not implemented in GNU C versions
earlier than 2.5. An alternative way to declare that a function
does not return, which works in the current version and in some
older versions, is as follows:
typedef void voidfn ();
volatile voidfn fatal;
`const'
Many functions do not examine any values except their arguments,
and have no effects except the return value. Such a function can
be subject to common subexpression elimination and loop
optimization just as an arithmetic operator would be. These
functions should be declared with the attribute `const'. For
example,
int square (int) __attribute__ ((const));
says that the hypothetical function `square' is safe to call fewer
times than the program says.
The attribute `const' is not implemented in GNU C versions earlier
than 2.5. An alternative way to declare that a function has no
side effects, which works in the current version and in some older
versions, is as follows:
typedef int intfn ();
extern const intfn square;
This approach does not work in GNU C++ from 2.6.0 on, since the
language specifies that the `const' must be attached to the return
value.
Note that a function that has pointer arguments and examines the
data pointed to must *not* be declared `const'. Likewise, a
function that calls a non-`const' function usually must not be
`const'. It does not make sense for a `const' function to return
`void'.
`format (ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK)'
The `format' attribute specifies that a function takes `printf' or
`scanf' style arguments which should be type-checked against a
format string. For example, the declaration:
extern int
my_printf (void *my_object, const char *my_format, ...)
__attribute__ ((format (printf, 2, 3)));
causes the compiler to check the arguments in calls to `my_printf'
for consistency with the `printf' style format string argument
`my_format'.
The parameter ARCHETYPE determines how the format string is
interpreted, and should be either `printf' or `scanf'. The
parameter STRING-INDEX specifies which argument is the format
string argument (starting from 1), while FIRST-TO-CHECK is the
number of the first argument to check against the format string.
For functions where the arguments are not available to be checked
(such as `vprintf'), specify the third parameter as zero. In this
case the compiler only checks the format string for consistency.
In the example above, the format string (`my_format') is the second
argument of the function `my_print', and the arguments to check
start with the third argument, so the correct parameters for the
format attribute are 2 and 3.
The `format' attribute allows you to identify your own functions
which take format strings as arguments, so that GNU CC can check
the calls to these functions for errors. The compiler always
checks formats for the ANSI library functions `printf', `fprintf',
`sprintf', `scanf', `fscanf', `sscanf', `vprintf', `vfprintf' and
`vsprintf' whenever such warnings are requested (using
`-Wformat'), so there is no need to modify the header file
`stdio.h'.
`section ("section-name")'
Normally, the compiler places the code it generates in the `text'
section. Sometimes, however, you need additional sections, or you
need certain particular functions to appear in special sections.
The `section' attribute specifies that a function lives in a
particular section. For example, the declaration:
extern void foobar (void) __attribute__ ((section ("bar")));
puts the function `foobar' in the `bar' section.
Some file formats do not support arbitrary sections so the
`section' attribute is not available on all platforms. If you
need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.
`constructor'
`destructor'
The `constructor' attribute causes the function to be called
automatically before execution enters `main ()'. Similarly, the
`destructor' attribute causes the function to be called
automatically after `main ()' has completed or `exit ()' has been
called. Functions with these attributes are useful for
initializing data that will be used implicitly during the
execution of the program.
These attributes are not currently implemented for Objective C.
`unused'
This attribute, attached to a function, means that the function is
meant to be possibly unused. GNU CC will not produce a warning
for this function.
`weak'
The `weak' attribute causes the declaration to be emitted as a weak
symbol rather than a global. This is primarily useful in defining
library functions which can be overridden in user code, though it
can also be used with non-function declarations. Weak symbols are
supported for ELF targets, and also for a.out targets when using
the GNU assembler and linker.
`alias ("target")'
The `alias' attribute causes the declaration to be emitted as an
alias for another symbol, which must be specified. For instance,
void __f () { /* do something */; }
void f () __attribute__ ((weak, alias ("__f")));
declares `f' to be a weak alias for `__f'. In C++, the mangled
name for the target must be used.
`regparm (NUMBER)'
On the Intel 386, the `regparm' attribute causes the compiler to
pass up to NUMBER integer arguments in registers EAX, EDX, and ECX
instead of on the stack. Functions that take a variable number of
arguments will continue to be passed all of their arguments on the
stack.
`stdcall'
On the Intel 386, the `stdcall' attribute causes the compiler to
assume that the called function will pop off the stack space used
to pass arguments, unless it takes a variable number of arguments.
`cdecl'
On the Intel 386, the `cdecl' attribute causes the compiler to
assume that the called function will pop off the stack space used
to pass arguments, unless it takes a variable number of arguments.
This is useful to override the effects of the `-mrtd' switch.
You can specify multiple attributes in a declaration by separating
them by commas within the double parentheses or by immediately
following an attribute declaration with another attribute declaration.
Some people object to the `__attribute__' feature, suggesting that
ANSI C's `#pragma' should be used instead. There are two reasons for
not doing this.
1. It is impossible to generate `#pragma' commands from a macro.
2. There is no telling what the same `#pragma' might mean in another
compiler.
These two reasons apply to almost any application that might be
proposed for `#pragma'. It is basically a mistake to use `#pragma' for
*anything*.